A magnetic tracking system includes a sender or transmitter which applies magnetic fields in space, and a sensor which detects the fields. Most commonly, the transmitter includes three coils directed along three mutually orthogonal axes. Typically, these coils are co-located, i.e., wound so that their centers are at or very near to the same point in space. The sensor or receiver typically includes a similar assembly of three coils or other sensors. The position and orientation of the receiver in the frame of reference of the transmitter can be determined from the set of relationships between each transmitter coil and each coil or sensor of the receiver. Thus, each transmitter coil is actuated to emit a magnetic field and the resulting magnetic field component in the direction of each receiver coil or other sensor is measured by the receiver.
Magnetic locating systems of this type are used in many applications. For example, the receiver may be attached to a surgical instrument so that the instrument can be tracked in the frame of reference of the operating room, or in the frame of reference of a previously acquired image of the patient. Also, a probe having a receiver mounted thereon can cooperate with a fixed transmitter so that as the probe is moved over the surface of a three-dimensional object, the contours of the object are deduced from the position of the probe. In still other applications, the sensor may be mounted on a part of a human body and used to measure the position and orientation of that body part relative to a frame of reference holding the transmitter. For example, a head-mounted sensor can be used to detect the direction in which a user has turned his or her head. In still other arrangements, a receiver may serve as a three-dimensional input tool for a computer or computer game. Although the various applications have been described with reference to a moving receiver and a fixed transmitter or sender, these can be reversed, so that the transmitter moves in the frame of reference of the sensor.
In a frequency-multiplexed AC system, each of the transmitter coils is driven at a different frequency, most commonly with a continuous sinusoidal signal at such frequency. If the sensor is in an arbitrary orientation relative to the transmitter, the axes of the sensor coils will not be aligned with the axes of the transmitter coils. In this case, each sensor coil detects the magnetic fields generated by all of the transmitter coils, so that each sensor coil delivers a composite coil signal which includes components at each of the transmitted frequencies.
The components in each sensor signal are separated from one another by techniques such as filtering or, most commonly, Fourier transformation of the sensor signal to yield a frequency domain representation. The separated components provide nine separate components, each of which represents the signal induced in one sensor coil by one transmitter coil. For example, there is a component SXY representing the signal induced on the sensor coil oriented in the X-direction of the receiver by the field from the coil oriented in the Y-direction of the transmitter. Similarly, there is a signal SXX representing the signal induced in the sensor coil oriented in the X-direction of the receiver by the field emitted from the coil oriented in the X-direction of the transmitter.
The position and orientation of the sensor in the frame of reference of the transmitter can be computed from the phase and amplitude of the various components. Algorithms for accomplishing this are shown, for example, in Jones, U.S. Pat. No. 4,737,794; Egli et al., U.S. Pat. No. 4,287,809; and Raab, U.S. Pat. No. 4,314,251, the disclosures of which are hereby incorporated by reference herein.
However, the phase of the received signal components relative to the phase of the transmitted field components must be known. In a “wired” system, the receiver is connected to the transmitter, so that the receiver operates in synchronism with the transmitter. Therefore, the receiver can directly determine the phase of the sensor signal components using the same timing reference employed by the sender.
In some applications, however, a wire or other direct connection between the sender and the sensor is undesirable or impractical. Therefore, wireless systems have been developed. Examples of wireless systems are disclosed in U.S. Published Patent Application No. 2005/0285590 (“the '590 Publication”) and in Anderson, U.S. Pat. No. 7,015,859 (“the '859 patent”). In a wireless system, the receiver is not synchronized with the transmitter unit. In general, the receiver cannot detect the phase of the signals used to drive the transmitter coils without ambiguity. For example as the sensor moves past the center point of the transmitter in one of the three directions constituting the frame of reference of the transmitter, the sign of a signal component reverses. This reverses the phase of the signal component in exactly the same manner as if the phase of the signal used to drive a transmitter coil was shifted 180° or π radians.
As disclosed in the aforementioned '590 publication, one solution to this problem is to calibrate the receiver while it is at a known position and orientation relative to the transmitter. By imposing this known start-up geometry on the system, the receiver can determine the phase relationship between the transmitter and the timing reference used by the receiver.
Another approach disclosed in the aforementioned '859 patent uses a drive signal for one or more transmitter coils, which includes a main drive frequency, and the second harmonic of the main frequency. The second harmonic is in-phase with the main frequency only once in each cycle of the main frequency. In the example shown in the '859 patent, the second harmonic has a positive-going peak synchronized with the positive-going peaks of the main signal, but not with the negative-going peaks of the main signal. Thus, by monitoring the sensor signal components at the main and harmonic frequencies, and noting when the two are in synchronization (as, for example, when both have a positive-going peak), the receiver can unambiguously determine the phase of the transmitter signal at the main frequency. While this approach can resolve the 0 or π phase ambiguity, it suffers from a serious drawback. Typically, each coil of a field transmitter is driven by a resonant circuit which includes the inductance of the coil and a capacitance. These components are selected so that the resulting circuit will be in-resonance at the main frequency, and hence, will efficiently generate magnetic fields when driven with a signal at the main frequency. These components are also designed to have a high “Q” value. Q is a measure of the quality of a resonant circuit. A circuit with a high Q value has a narrow bandwidth. That is, the circuit can be driven very efficiently within a narrow band of frequencies encompassing the resonant frequency of the circuit, but is very inefficient at frequencies outside this band. By definition, the second harmonic is at a frequency double the main frequency. If the resonant circuit must operate at both of these frequencies the resonant circuit have a relatively low Q, and hence, a large bandwidth. This sacrifices efficiency at the main frequency, leading to higher power consumption. Although a separate resonant circuit can be added to generate fields at the second harmonic, this considerably increases the complexity and cost of the transmitter.
Accordingly, further improvement would be desirable.